As is well known, electromagnetic flowmeters measure the volumetric flow rate of an electrically conductive liquid flowing through a pipe; thus, per definitionem, the liquid volume flowing through a pipe cross section per unit time is measured.
The flowmeter has a, usually nonferromagnetic, flow tube which is connected into the pipe in a liquid-tight manner, e.g., by means of flanges or threaded joints. The portion of the flow tube which contacts the liquid is generally electrically nonconductive, so that a voltage induced in the liquid according to Faraday's law of electromagnetic induction by a magnetic field cutting across the flow tube will not be short-circuited.
Therefore, metal flow tubes are commonly provided with a nonconductive lining, e.g., a lining of hard rubber, polyfluoroethylene, etc., and are generally nonferromagnetic; in the case of flow tubes made completely of plastic or ceramic, particularly of alumina ceramic, the nonconductive lining is not necessary.
The magnetic field is produced by means of a coil assembly comprising at least two field coils, each of which is positioned on the flow tube along a diameter of the latter. The field coils may be air-core coils or coils with a core of soft magnetic material.
To ensure that the magnetic field produced by the field coils is as homogeneous as possible, the coils are, in the most frequent and simplest case, identical and electrically connected in series, so that in operation they can be traversed by the same excitation current. It is also known to cause the same excitation current to flow through the field coils alternately in the same and the opposite direction in order to be able to determine a flow profile and/or a liquid level in the pipe, see U.S. Pat. No. 5,493,914, or in order to be able to measure the viscosity of non-Newtonian fluids, see U.S. Pat. No. 5,646,353.
The excitation current just mentioned is produced by control electronics; it is regulated at a constant value of, e.g., 85 mA, and its direction is periodically reversed; this serves in particular to largely compensate electrochemical interference voltages developed at the electrodes. The current reversal is achieved by incorporating the field coils in a so-called T network or a so-called H network; for the current regulation and current reversal, see U.S. Pat. No. 4,410,926 or U.S. Pat. No. 6,031,740.
The aforementioned induced voltage is picked off by means of at least two galvanic, i.e., liquid-wetted, electrodes, or by means of at least two capacitive electrodes, i.e., two electrodes disposed in the wall of the flow tube, for example, which in the most frequent case are arranged at diametrically opposed positions such that their common diameter is perpendicular to the direction of the magnetic field and, thus, perpendicular to the diameter on which the field coils are located. The induced voltage is conditioned by means of evaluation electronics to obtain a volumetric flow rate signal, which is recorded, displayed, or further processed.
Electromagnetic flowmeters measure volumetric flow rate with optimum accuracy if the flow in the flow tube is uniformly turbulent. Under this condition of uniform turbulence, each flowmeter is calibrated by the manufacturer, and the values of the so-called calibration factor and the zero drift, which are determined during this calibration, are electronically stored in the flowmeter.
To ensure that after its sale, the flowmeter can be operated with this accuracy in the field, the manufacturer generally specifies an undisturbed inlet section, which is a straight tube length and must be present or be inserted between the flowmeter and a spot of the pipe which disturbs or may disturb the uniform turbulence. Such pipe spots are, for instance, elbows, valves, etc.
During operation of the flowmeter, however, the uniformly turbulent flow profile thus generated may become nonuniform despite the inlet section as a result of unforeseeable events or changes in the liquid. This makes the measurement results inherently more inaccurate and in the worst case even may invalidate the measurement result without this being noticeable.
It is therefore desirable to detect such accuracy-reducing events during measurements, i.e., to derive a corresponding error signal, which then is displayed, triggers an alarm, or serves to correct the measurement result, etc.
To determine the flow profile, but particularly to compensate disturbances in the flow profile, U.S. Pat. No. 5,325,724 proposes to cause excitation currents to flow through both field coils, which produce equidirectional, but temporarily differently strong partial magnetic fields. Investigations have shown, however, that these solutions proposed in U.S. Pat. No. 5,325,724 do not produce the desired effect, namely a significantly asymmetric magnetic field, particularly in the area of the measuring electrodes.